<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <style>
    *{
      margin: 0;
      padding: 0;
    }
    html, body{
      height: 100vh;
    }
    .content{
      height: 100vh;
      background-color: red;
    }
    .box1{
      height: 100px;
      background-color: green;
    }
    .box2{
      background-color: yellow;
      height: calc(100% - 100px);
    }
  </style>
</body>
</html>